Structures may be bonded together using a variety of adhesives. One adhesive that is relatively easy to apply while still providing good bonding properties is an isocyanate-containing polyurethane adhesive, which may be sprayed onto a substrate and then subsequently cured to provide a strong bond. However, the use of such polyurethane adhesives in lamination processes represents an occupational health risk and major precautions must be taken during operation of the process in order to protect the personnel involved. An alternate adhesive under development is a moisture curable 100% solid hot-melt polyurethane adhesive, but it is understood that such adhesives developed to date suffer from stability problems and may not have the desired bond strength for many end uses. It would be desirable to have alternate processes that do not use polyurethane adhesives.
The cross-linking of a polymer is used to alter the properties of the polymer. Thus, it is possible to fabricate a polymer into a shape, including in the form of an adhesive, and then to cross-link the polymer to improve the end use properties. In one such method, vinyl silanes, e.g., vinyl trimethoxysilane (VTMS) and vinyl triethoxysilane (VTES), are grafted onto polymers. The grafted polymers are formed into a desired shape under molten conditions and the resultant article is subsequently subjected to moisture, e.g., steam or atmospheric moisture, whereupon the silane residue forms cross-links between polymer chains. The grafting reaction of the vinyl silane onto the polymer is usually carried out in the presence of an organic peroxide, and the cross-linking reaction with moisture is usually carried out in the presence of a cross-linking catalyst.
An adhesive for heat-sealing of metals and plastics is disclosed in Japanese Patent 1,161,079 of Showa Electric Wire KK, which issued Jun. 23, 1989. The adhesive composition disclosed is composed of 100 parts by weight of one or more of polyethylene, polypropylene, ethylene/vinyl acetate copolymer or ethylene/ethyl acrylate copolymer grafted with maleic acid and/or acrylic acid in amounts of 0.5-10 wt. %, and 0.1-5 parts by weight of a silane coupling agent. Preferably the adhesive composition is blended with 0.05-3 wt. % of an organic peroxide that has a half-life of one minute at a temperature in the range of 100.degree.-180.degree. C. The adhesive is stated to have good adhesion and heat resistance in the bonding of plastics to metal and plastics to plastics, but would not be stable under melt conditions and hence not useful as a hot melt adhesive.
As used herein, melt viscosity is measured at 190.degree. C. on a Brookfield viscometer at a shear rate of 0.35 sec.sup.-l.